(1) Field of the Invention
The present invention relates to (1) a displacement detecting circuit for controlling the posture of a running vehicle or for immediately and accurately detecting displacement in the level of a precision surface plate which supports a holography set, an electron microscope, a semiconductor manufacturing device or any other precision measuring device, (2) a novel control circuit with the above detecting circuit incorporated into a variation control circuit which maintains or improves the precision of devices by preventing or suppressing microvibrations transmitted from the floor, (3) a variation control circuit including the above control circuit and having a displacement detecting function, and (4) an active control precision damping table including the above circuit, which table supports a holography set, an electron microscope, a semiconductor manufacturing device or other precision measuring device.
(2) Description of the Prior Art
(1) A known example of a displacement detecting circuit utilizing an encoder will be described. In this example, the encoder has a reference scale attached to a reference surface and a movable scale attached to a movable side. The encoder outputs a pulse signal corresponding to an amount of movement of the movable scale from zero point on the reference scale. The amount of movement is detected through a count of the number of pulses. The precision and resolution in detecting the amount of movement are limited since they are dependent on the mechanical precision and mechanical construction of the pulse motor. To improve the detecting precision, the encoder per se must be increased in size. There has also been the problem of detection of movement being limited by the encoder capacity.
(2) A known variation control circuit simply detects a disturbance transmitted from the floor to a measured object, and outputs a signal corresponding to the disturbance to a drive circuit for controlling the disturbance. There has been no measure taken to deal with level variations due to load displacement or a shift in the center of gravity on the vehicle or the surface plate.
(3) Controls of the displacement and shift require a level sensor and a variation sensor, which would complicate the circuit construction.
(4) Known damping tables have all relied simply on a damping system employing support by air springs. This system produces a conspicuous damping effect in a high frequency region but allows resonance in a low frequency region. When an impact occurs, it is not immediately absorbed but produces a very large amplitude as shown in FIG. 11. There is a problem of slow damping speed with the shock gradually attenuating thereafter to a certain level. There have been proposals to effect active control by means of a control valve or the like, but these proposals are hardly practicable since they fail to deal with level variations due to load variation or shift.